A slice of basement in the western margin of the Appalachian orogen, Saint-Malachie, Quebec

A slice of basement in the western margin of the Appalachian orogen, Saint-Malachie, Quebec

A. VALLI~RES Ministtre des Richesses Naturelles, Gouvernement du Que'bec, Quebec (Qut.), Canada CIS 4N6

AND

C . HUBERT AND C. BROOKS Departernent de Gtologie, Universite' de Montrtal, Montrtal (Que'.), Canada H3C 3J7

Received December 5 , 1977 Revision accepted March 14, 1978

Slivers of gnei5r;es and amphiholites (termed the Sainte-Marguerite Complex) having a minimum isotopic age of lHHl Ma are interpreted to represent thtvst stices of Grenvillian-type basement within the wePtern margin of the internal domain of the Appalnchidn orogen. These slices conslitute the basemen1 to a series of spilitized volcanic rocks and interhedded shallow- water sandstones (termed the Montagne de Saint-Anselrne Formatjon) which are interpreted to he Precamhnan to Camhri;~n.(?) sift-related precursors to the filling of the Appalachian mobile belt.

On interprbte les copeaux de gneiss et d'amphiholites (appeKs complexe de Sainte-Marguerite) d'8pe mdiom0trique m~nirnurn de 900 Ma comme representant des ecailles de chevauchement d'un ~ o c l e de type grenvillren dans la bordure ouest du domaine interne de I'orogene des Appalaches. Ces ecaillesconblituent un socfe p u r une &rie de ruches volcaniques spilitisees et de gres interlitkg d'eau peu profondc (appeler formation de la Montagne de Saint-Anselrne) qu'on interprkte comrne des roches qui. du Precambrien ail Cambrien(?), ont prectdk dans la zone de nft le remplissage de la zone mobile des Appalaches.

[Traduit par le journal] Can. J . Earth SCI., IS, 1242-1249 (1978)

Introduction Cambro-Ordovician rocks along the western

margin of the Quebec Appalachians have recently been divided into (i) an external domain comprising an outer belt of thrust-imbricated structures and an inner belt of gravity-emplaced nappes, and (ii) an internal domain of metamorphosed sedimentary and volcanic rocks within large-scale imbricated nappes (Fig. 1). The main features of these domains have been previously discussed (St-Julien and Hubert 1975), and it was emphasized that the principal difference between the domains is the in- volvement of basement in the tectonism of the internal zone whereas in the external domain the tectonism is confined to the cover rocks. Further, two types of basement are recognized in the internal domain, one with gneissic rocks and the other comprising ophiolite suites.

The gneissic rocks crop out in the Saint-Malachie area (Fig. I), 30 km southeast of Quebec City (Na- tional Topographic System, sheet no. 21Ll10). They are lithologically similar to those Precam- brian rocks that occur elsewhere along the western margin of the Appalachian orogen, where they constitute the core of anticlinoria (e.g. the Blue Ridge, Berkshire Massif, Hudson Highlands, Green Mountain, etc.). In all these occurrences, the crystalline rocks which are characterized by isotopic

dates of about 800-1OOOMa most probably formed as part of the Grenville structural province of the nearby Canadian Shield.

The Saint-Malachie occurrence is the first occurrence of Precambrian rocks in the Quebec section of the western margin of the Appalachian orogen. Because of this and because from the viewpoint of structural setting and lithology, the St-Malachie rocks differ markedly from the Precambrian sequences of the central (Chain Lake Massif) and northern sections of the orogen elsewhere in Canada and U. S.A., we have investigated the rocks in some detail. The purpose of this paper is to report our findings on this occurrence and to dis- cuss the geological implications in the light of the tectonic evolution of the Appalachian orogen.

The principal geological features of the Saint- Malachie area are given in Fig. 2. The main feature of this figure is the Richardson fault which marks the western boundary of the internal domain. The 'Precambrian' rocks described in this paper occur along this major structure and are associated with Precambrian-Cambrian(?) lavas. The other nota- ble feature of this region is the Montagne de Saint- Anselme Klippe consisting of a well-exposed sequence of interstratified lavas and sandstones some 1 630 m in thickness. Details of the stratigraphy are summarized in Table 1, and Fig. 3 shows some 1

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FIG. 1. Geographic setting of the Saint-Malachie area and main tectonic domains of the Quebec Appalachians (after St-Julien and Hubert 1975). c = Carboniferous; mi = Mictaw Group; ma = Maquereau Group.

700:5,.,,. structural details of the Richardson fault together with the lithological units in each of the imbricated blocks constituting that fault zone. Because of the

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - unique properties encountered in these Precam-

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - brian rocks, we refer to them as the Sainte- - - - - - - - - - - - - Marguerite Complex. The name is derived from the

village of Sainte-Marguerite-de-Dorchester (longitude 70°56', latitude 46"3 1'). The associated, Pre- cambrian-Cambrian(?), dominantly igneous sequence is given a formal stratigraphic name, Mon- tagne de Saint-Anselme Formation, derived from the 630m exposure at the Montagne de Saint- Anselme (longitude 70"59', latitude 46"3Sr). This is because future studies will attempt chemostratigraphic correlations of these lavas with other occurrences in the western part of the internal domain. The geochemistry of these lavas figures in our interpretation of the early history of the Ap- palachian orogen and is also reported in this paper.

The Precambrian Sainte-Marguerite Complex This complex consists of three slivers, one of

interlayered gneissic granites with pegmatites and Selnr uorn Fnrmargoa amphibolites, one of gneissic granites and pegma-

t i t e ~ , and one of metaquartzite (legend, Fig. 3, slivers A, B, and C, respectively).

46"30 Sliver A is 3 km in length and 600 m in width. It FIG. 2. Geological map of the western part of the Saint- includes a 70m thick amphibolite band which is

Malachie region (after Vallieres 1971). oriented northeast-southwest. This band is com-

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1244 CAN. J. EARTH SCI. VOL. IS, 1978

S a ~ n ~ e M a r g u e r n e Complex

,,' l ~ l h o l o g ~ c a l conlac1

A, s ~ r ~ k e and d ~ p of b e d d ~ n g

.C l a c ~ n g of s l r a l ~ g r a p h ~ c secllon

FIG. 3. Detailed geology of the Richardson fault zone between Sainte-Marguerite-de-Dorchester and Etchernin River, Saint- Malachie.

posed of layers of amphibolite (1 cm to 15 m in thickness) alternating with layers of gneissic granites. The amphibolites show a banding foliation that corresponds to thin green layers composed of chlorite, quartz, apatite, sericite, epidote, sphene, and leucoxene, and thin black layers composed of brown hornblende with accessory apatite. In these bands original plagioclase has been retrograded to chlorite and epidote. The hornblendes from the amphibolites of Sainte-Marguerite Complex have a composition of SiO,, 48.6%; TiO,, 2.3%; Al,O,, 10.2%; FeO, 14.7%; MgO, 10.5%; CaO, 10.5%; Na,O, 1.8%; and K,O, 1.3% (mean of two analyses), which is very similar to that of hornblendes from the granulite facies rocks of the Grenville in the Adirondacks. Using the TiO, ver- sus total alkali relationships proposed by Leake (1965) we conclude on comparative compositional grounds (Fig. 4) that hornblendes in the amphibolites of the Sainte-Marguerite Complex were in granulite facies grade before retrogression. The gneissic granites in sliver A have a faint gneissosity resulting from alignment of chlorite, are pale grey, and have granoblastic texture. Thin mafic dykes

cut this gneissosity and foliation in both gneissic granites and amphibolites, although they are too fine grained and altered to classify mineralogically. Typically they range from 10 to 100 cm in thickness and are exclusively restricted to the Sainte- Marguerite Complex.

The gneissic granites and pegmatites in sliver B are generally similar to those of sliver A. They are pink to white, and their overall composition is quartz (45%), potash feldspar (30%), albite (15%) with biotite (bleached), ilmenite, magnetite, and hematite, and accessory apatite, chlorite, sphene, leucoxene, epidote, calcite, tourmaline, and garnet. The degree of alteration is pronounced, par- ticularly with regard to feldspars which in some places are strongly altered to chlorite and sericite matrices within which isolated quartz grains are evident. No evidence of a granulite facies metamorphism is now visible in these rocks and they are apparently a retrograde assemblage in upper greenschist facies.

The metaquartzites of sliver C are massive and composed almost entirely of quartz with minor chlorite, biotite (bleached), hematite, ilmenite, and zircon. Contact between quartz grains is sutured and in some places microstylolitic. These grains are elongated with a mean long axis dimension of 5 mm (some range to 20mm). Many of them show un- dulose extinction and microscopic strain planes.

Thus, in resume, the complex shows lithological similarities to gneisses of the nearby Grenville province and it contains evidence of at least two stages of metamorphism. The question of whether these rocks are of Grenvillian age prompted a geochronological investigation of two of the units in which datable minerals were observed, i.e., the amphibolites, and the biotite-bearing gneissic granites of sliver A. From these rocks respectively, amphibole and biotite were separated and the former dated by conventional K/Ar techniques, the latter by Rb/Sr techniques. The results are given in Table 2. The overall agreement in age between the coarse-grained amphibole and the biotite, which show quite different susceptibilities to metamorphism, strongly suggests that the 900-950Ma age represents a reliable minimum value for the last metamorphic event. Thus the rocks have been sub- ject to an event of Grenvillian age and are hence unquestionably Precambrian. It is worth noting that the classic orogenic events of the Paleozoic (Taconic and Acadian) have not reset the ages observed in the Sainte-Marguerite Complex. This is especially surprising in view of the sample locations (within the Richardson fault zone) and the age of the Richardson fault (stated to be Taconic on geological grounds (St-Julien and Hubert 1975)).

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VALLI~RES ET AL.

TABLE 1. Stratigraphic relationships in the western part of the Saint-Malachie region (for lithologies see legend of Fig. 3)

INTERNAL DOMAIN EXTERNAL DOMAIN

LOWER ORDOVICIAN

RIVI tRE OUELLE FM SAINTE H ~ N ~ D I N E FM

KAMOURASKA FM -------- - 3

UPPER 2 Z SAINT DAMASE FORMATION Q - - = m

2 - U SAINT ROCH FORMATION

LOWER

----------------- FORMATION - - - - - - - - - - -

Z HADRYNIAN 9 - ------ ---- z SAINTE MARGUERITE

HELlKlAN CL COMPLEX 0

This implies that the time of retrogression of the Sainte-Marguerite Complex is of Grenvillian age.

Having established the Precambrian age of the Sainte-Marguerite Complex we can correlate it with other Precambrian complexes along the west- em margin of the Appalachians of Canada and U.S.A. Most of them like the Sainte-Marguerite Complex show similar structural position, similar lithological make-up, and similar retrograde metamorphic features. The cores of these complexes have radiometric ages similar to those of the Sainte-Marguerite Complex although interpretation has often been complicated by younger ages obtained in the peripheral zones using conventional KlAr incremental heating techniques (e.g., Dallmeyer et al. 1975).

Lavas of the Montagne de Saint-Anselme Formation These lavas occur in several separate slivers

within the Richardson fault zone and within the Mippe at the Montagne de Saint-Anselme (Fig. 2). They represent a 630 m thick sequence consisting, at Montagne de Saint-Anselme, of 16 lava flow units and interlayered sandstones. Four kilometres northeast of Sainte-Marguerite-de-Dorchester, there are 18 lava flow units and interlayered sandstones in the sequence. Each unit of lava flows is generally 20-100m thick and that of sandstones from 20 to 130m. Some of the flows are pillowed. Others are composed almost entirely of scoriaceous lavas.

The sandstones are arkosic in composition but some are coarse grained, others are very fine grained. The former are flesh colored, the latter grey-green. The beds are generally massive; in some places, they are finely laminated and with rare

0 0

SAlNTE MARGUERITE . C O M P L E X

I N THE A D I R O N D A C K S

GRANULITE FACIES 0

AMPHlBOLlTE FACIES 0

FIG. 4. Comparative alkali-titania compositions of hornblende~ from amphibolite and granulite facies in the Adiron- dacks and from amphibolites in the Saint-Marguerite Complex.

graded-bedding and crosszbedding (torrential-type 30 cm to 1 m in height). The coarse-grained arkosic sandstones are composed of 25-40% feldspar of which half is potassic. The plagioclase is An,- An,,. Quartz is the dominant remaining constituent with minor magnetite, ilmenite, hematite, sphene, leucoxene, garnet, crystalline lava fragments, shards of volcanic glass, and augite crystals. This sandstone displays poor sorting, 0.1-4 mm grain size, subangular to subrounded grains of low sphericity, and less than 5% chloritized matrix. Mineralogical maturity is low, indicating short transportation distances and a provenance involving volcanic material. The very fine-grained arkosic sandstones are composed of angular quartz and feldspar grains of low sphericity, floating in a chloritized matrix (50 volume percent of the rocks). The grains are silt and very fine sand-size (0.2 mm). Typically they consist of quartz (25%), albite

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CAN. J. EARTH SCI. VOL. 15, 1978

TABLE 2. Geochronological data for four samples from the Sainte-Marguerite Complex

Biotite (Rb/Sr) - Sample Rb ( P P ~ ) Sr ( P P ~ ) 87Rb/86Sr 87Sr/86Sr Age (Ma)

AV-616C 367 24.7 42.8 1.272 954 5 40 - -

Amphi boles (K/Ar)

Radiogenic Atmospheric Sample wt.% K argon (atom/g) 40Ar (73 Age (Ma)

NOTE: Average for fine-grained material (MGA - 2A, 28): 863 Ma. Age for coarse-grained material: 900 Ma. Assumed initial Sr ratio = 0.705.

(lo%), potash feldspar (3%), sphene (5%), and opaques (5%) with accessory apatite and garnet.

The lavas are of basaltic composition containing plagioclase (60-SO%), clinopyroxene (10-20%), and olivine (0-5%), with subophitic textures, minor magnetite, ilmenite, sphene, and apatite. The rocks are altered (plagioclase is Ano-An,,) and the only primary mineral is a magnesian augite (2VZ: 44-50", Z,c: 37-40", optical plane 010).

Examination of the pillowed lavas of the Mon- tagne de Saint-Anselme Formation in thin section, indicates that they have certain properties charac- teristic of spilite (e.g., clear limpid albite). Because the chemistry of spilites is generally suspect they are not usually regarded as reliable indicators of mode of tectonic environment, However, useful geochemical data can under certain conditions be obtained from spilite.

Firstly, studies of similarly altered rocks (Gelinas and Brooks 1974; Brooks and Gelinas 1977; Gelinas et al. 1977) indicate that whereas the alkali chemistry may be modified, primary trends and abundance levels of other elements may yet be preserved. For example, Archean lavas commonly display iron-enrichment trends on the FMA diagram, and it is quite probable that these are primary features of the lavas. The scattering of data about these trends, however, appears related to the level of alkali content in the individual samples, and it seems that in a general way, the degree of alkali addition during spilitization correlates with the dis- persion of data about the iron-enrichment trend. This qualitative observation is only really useful where spilites define good iron-enrichment trends that do not correlate with water contents. These cases can indicate that no major addition of alkalis accompanied spilitization which must then have been essentially an isochemical rearrangement of sodium and calcium during low-grade metamorphism.

Secondly, the primary chemical nature of altered

WEIGHT % FIG. 5. P20S-TiO, diagram for lavas of the Montagne de

Saint-Anselme Formation.

lavas can be interpreted in terms of the abundance levels of those elements that are not mobile during spilitization and low-grade metamorphism. Recent studies (e.g., Hart et al. 1974) have demonstrated that TiO, and Sr show only minor changes during submarine alteration, whereas SiO,, Al,O,, and CaO are lost from the basalt, and total Fe, K,O, and Rb increase.

Fifteen samples representative of the lavas were selected for analysis and the results together with normative compositions are presented in Table 3 (for sample locations, see Appendix). As recom- mended for volcanics containing secondary mineral assemblages, the analyses are presented anhydrous, with the appropriate volatile content (CO, + H20) also indicated. The major and trace-

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TA

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1 2

3 4

5 6

7 X

9 10

11

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13

1

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SiO

, 49

.90

49.0

5 A

1203

14

.07

14.7

1 F

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14.7

5 13

.64

MgO

5

.32

5

.92

C

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5

8.45

N

a,O

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33

3.3

9

K2

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2.40

1

.74

T

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2.35

2.

54

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.33

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M

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0.31

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1248 CAN. J. EARTH SCI. VOL. 15, 1978

element analyses are X-ray fluorescence values (for details see Gunn and Watkins (1970)) and the volatile contents were determined by gas chromatog- raphy at the Universite de Montreal (for details see Brooks et al. (1969)).

Examination of the data in Table 3 reveals that the lavas are quite uniform in composition, being basalts within the SiO, range 49-52 wt.%. The con- stancy of composition confirms the field observa- tions that differentiates are lacking. None of the basalts are quartz-normative, although 2 of the 13 are nepheline-normative, and the rest are hypersthene-normative. The TiO, abundances are high, uniform, not correlated against H,O but correlated against P205 (Fig. 5). This suggests that the observed values are probably good indicators of the primary levels in the Montagne de Saint-Anselme Formation lavas. The TiO, content of continental tholeiites is quite variable but the values are typically less than about 1.7% (except from the more extreme potash and Mg-rich varieties; e.g., Rhode- sian Karroo basalts (Jamieson and Clarke 1970)). Continental alkali basalts, however, have TiO, contents ranging from 1.4 to 3.6% with a mean value of 2.3 k 0.6% (Schwazer and Rogers 1974). This difference could be used to suggest that the Montagne de Saint-Anselme lavas were of alkali basalt rather than tholeiitic affinity. A similar ar- gument can be based on the relatively elevated P205 values in these lavas, although the suggestion requires additional support. Unfortunately trace- element data for the Montagne de Saint-Anselme lavas is not unambiguous, although the Sr (200-300 ppm) and Rb (20-60 ppm) levels are not consistent with normal tholeiite compositions.

The geochemical nature of the Montagne de Saint-Anselme Formation lavas indicates that they are undersaturated basalts of generally uniform composition. This composition is not uniquely diagnostic of alkali basalt chemistry although the non-mobile elements Ti, and Sr together with P, indicate that neither are the lavas tholeiites similar to those of plateau basalt occurrences. We conclude that the Montagne de Saint-Anselme Forma- tion basalts are most likely of alkaline affinity or at the least transitional between tholeiite and alkali basalt. Because the compositional nature of these lavas has important implications about the early rifting stages associated with the late Precambrian development of the Appalachian mobile belt, further work involving analysis of the nonmobile trace elements is planned for these lavas (e.g., using the combined Ti, Sr, Zr, Y, and Nb dis- criminators proposed by Pearce and Cann (1973)).

Discussion Evidence from lithological and field mapping ob-

servations, supported by radiometric ages, suggests that the Sainte-Marguerite Complex of Dor- chester County, Quebec, represents the extension in that region of rocks of the Grenville province. As such it would constitute a basement complex of Precambrian age (>900 + 50 Ma) that could be used to delineate the western boundary of the internal domain. In any event, we have conclusive evidence for the participation of Precambrian basement in the evolution of the internal nappes. Furthermore our data provide additional verification of earlier suggestions (Rodgers 1970; Wynne-Edwards 1972) that rocks of the Grenville province extend well under the zone now occupied by the Appalachian mobile belt. The restriction of the Sainte- Marguerite Complex to a narrow zone abutting the Richardson fault suggests that additional exposures of Precambrian basement may also be recognized elsewhere along this structure. Clearly the tectonic setting that we have deciphered in this part of Quebec is remarkably similar to tectonic settings observed elsewhere in the Appalachian orogen and we suggest that the following structural entities represent the western boundary of the internal domain extrapolated from Quebec southwards: Green Mountain, Hudson Highlands, Berkshire, Reading Prong, and Blue Ridge (cf. Rankin 1976~). The Quebec occurrence differs from these only in being more strongly dissected and of smaller scale. This could be advanced as an explanation for the appar- ent absence of Precambrian outcrops in Quebec, and using the Sainte-Marguerite Complex as a model, many additional Precambrian slivers may eventually be recognized along the western margin of the Appalachians.

The sandstones of the Montagne de Saint- Anselme Formation contain sedimentary structures indicating a shallow-water depositional environment. The interdigitated and pillowed lavas were therefore also deposited under shallow-water conditions and this is supported by the numerous scoriaceous bands within the lava sequences. This contrasts with other Appalachian sequences where the sediments are generally interpreted as being deep-water (Atlantic-type continental margin (St- Julien and Hubert 1975)). If it is accepted that the chemistry of these lavas indicates an igneous association of alkaline affinity then we can interpret the Montagne de Saint-Anselme Formation as constituting a precursor to the filling of the Appala- chian mobile belt, with the lavas representing the products of early-rifting.

This supports similar conclusions previously advanced by Williams and Stevens (1969, 1974), Strong and Williams (1972), and Rankin (1976a,b) that the earliest volcanic sequences interlayered with shallow-water or continental sediments may

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well represent the sites where substantial crustal Pitcher and G. W. Flinn. John Wiley & Sons, New York, NY. separation has taken place along the western margin of the Appalachian orogen (e.g., the Lighthouse

; Cove Formation, Newfoundland, and the Catoctin i Formations, Virginia). Because the Montagne de I Saint-Anselme Formation is thin we can further i suggest that either the formation represents the I western margin of the Appalachian mobile belt, or

i '

that it was deposited on a topographic high of Grenvillian basement. The existence of down- faulted blocks of Appalachian material further to the west (St. Lawrence graben) negates the former and we favor in this instance the alternate pos- sibilities. It could be speculated that pre-Ap- palachian topographic highs were decapitated during the nappe activity resulting in field occurrences as typified by the Sainte-Marguerite Complex.

I Acknowledgements We wish to thank the Department of Natural

Resources for supporting field work (Hubert and Vallieres), Dr. B. Gunn for geochemical analyses, R. Coy-Yll for microprobe analyses, and J. Barton for K/Ar ages. This study was supported in part by the National Research Council of Canada (No. A5242 to C. Hubert and No. 5581 to C. Brooks).

BROOKS, C., and GELINAS, L. 1977. The relationship of base metal mineralization to chemostratigraphic divisions in volcanic rocks of Canadian Archean greenstone belts. IREM- MERI, Research Paper 77-1, pp. 1-19.

BROOKS, C., HART, S. R., KROGH, T. E., and DAVIS, G. L. 1969. Carbonate contents and Srs71Srs6 ratios of calcites from Archean metavolcanics. Earth and Planetary Science Letters, 6, pp. 35-38.

DALLMEYER, R. D., SUTTER, J. F., and BAKER, D. J. 1975. Incremental 40Ar139Ar ages of biotite and hornblende from the northeastern Reading Prong: their bearing on Late Pro- terozoic thermal and tectonic history. Geological Society of America Bulletin, 86, pp. 1435-1443.

GELINAS, L., and BROOKS, C. 1974. Archean quench texture tholeiites. Canadian Journal of Earth sciences, 11, pp. 324-340.

I GELINAS, L., BROOKS, C., PERRAULT, G., CARIGNAN, J., TRUDEL, P., and GRASSO, F. 1977. Chemo-stratigraphic divisions within the Abitibi volcanic belt, Rouyn-Noranda dis- trict, Quebec. Geological Association of Canada, Special Paper 16, pp. 265-295.

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Appendix Samples are located by coordinates using the

Universal Transverse Mercator Grid, Zone 19

Sample No. Coordinates

Can

. J. E

arth

Sci

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

WA

ST

AT

E U

NIV

LIB

RA

RIE

S on

12/

15/1

4Fo

r pe

rson

al u

se o

nly.

Documents

A slice of basement in the western margin of the Appalachian orogen, Saint-Malachie, Quebec